This invention relates generally to turbine engines, and, more particularly, to cooling systems for turbine engines.
A gas turbine engine typically includes a multi-stage axial or centrifugal compressor, a combustor, and a turbine. Airflow entering the compressor is compressed and directed to the combustor where it is mixed with fuel and ignited, producing hot combustion gases used to drive the turbine. As a result of the hot combustion gases entering the turbine, compressor air may be channeled through a turbine cooling circuit and used to cool the turbine.
Compressor bleed may be extracted through a cooling system and used as a source of cooling air for the turbine cooling circuit. However, extracting large amounts of cooling air from the compressor may affect overall gas turbine engine performance. To minimize a reduction in engine performance, the cooling system may use fuel flowing through a heat exchanger to absorb heat from the compressor bleed air, but the use of fuel as a heat sink causes potential fire safety and fuel deposit problems. As hot compressor bleed air passes through the heat exchanger, fuel flow absorbs heat from the compressor bleed air.
However, as fuel is heated, often carbon, gum, and coke deposits form within tubing used to transport fuel through the heat exchanger. Over time, such deposit agglomeration blocks individual tube passages causing an increase in cooling air temperature. Because of the increased cooling air temperature, less heat is absorbed from compressor bleed air and turbine components receiving compressor bleed air are cooled ineffectively. As a result, such components are subjected to increased low cycle fatigue, LCF, stresses and increased thermal stresses. Furthermore, because turbine components are cooled less effectively, overall engine performance, life, and reliability decreases.
In an exemplary embodiment, a selectively operable cooling system reduces fuel gum deposits within the cooling system to a gas turbine engine when the gas turbine engine is operated above a predefined percentage of engine rated power. The cooling system includes a recirculating loop including a plurality of heat exchangers in fluid communication with the recirculating loop. A first heat exchanger is an air-fluid heat exchanger that uses heat transfer fluid to cool cooling air supplied to the gas turbine engine. A second heat exchanger is a fluid-fuel heat exchanger that uses combustor main fuel flow to cool the heat transfer fluid circulating in the recirculating loop.
During gas turbine engine operation above a predefined percentage of engine rated power, the cooling system is placed in operation when sufficient deposit dissipation forces are available within the second heat exchanger. Fuel passing through the second heat exchanger flows through fuel paths to facilitate heat transfer from the heat transfer fluid to the fuel. As a result of the flow through the fuel paths, the fuel flow develops high fluid turbulent forces and fluid shear forces. Such fluid forces reduce fuel deposits within the second heat exchanger.